Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell stack of the fuel cell power system, individual fuel cells provide channels through which various reactants and cooling fluids flow. Movement of water from the channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell assembly. Drag forces pull the liquid water through the channels until the liquid water exits the fuel cell through the outlet manifold. However, when the fuel cell is operating at a lower power output, the velocity of the gas flow is too low to produce an effective drag force to transport the liquid water, and the liquid water accumulates in the flow channels.
A further limitation of utilizing gas flow drag forces to remove the liquid water is that the water may encounter various surface irregularities with high or low surface energy or the water may encounter pinning points on the flow channel surfaces. Because the drag forces may not be strong enough to effectively transport the liquid water, the pinning points may cause the water to accumulate and pool, thereby stopping the water flow. Such pinning points are those commonly located where channel inlets and channel outlets meet the fuel cell assembly manifold.
At the outlet aperture of each fuel cell plate, water must overcome a pinning force on the edge thereof. Moreover, for a hydrophilic surface, there is a capillary force that acts in the direction of a lower radius of curvature of the interface of the liquid water and water vapor. Liquid water and water vapor tend to flow from a region that produces a water vapor gas/liquid water interface having a radius of curvature, such as manifolds, for example, to a region producing a gas/liquid interface having a smaller radius of curvature, such as the flow channels, for example. The radius of curvature of the gas/liquid interface will vary based on the size of the region in which the interface is formed. For example, as the width, or other dimension, of the region increases, the radius of curvature of the interface will also increase. The capillary force is represented by the equation:ΔP=Pnonwetting−Pwetting=[(2σ)/R]*cos θWhere:
Pnonwetting=pressure in gas (air or hydrogen) phase
Pwetting=pressure in liquid (water) phase
σ=liquid surface tension
θ=static contact angle
R=radius of curvature of gas-liquid interface
For a fuel cell bipolar plate with hydrophilic surface (i.e., θ<90°), residual water may be pulled from the outlet manifold and into the flow channels in the absence of reactant gas flow. Additionally, in cold operating conditions, condensation may form in the inlet manifold upstream from the inlet apertures of each fuel cell plate. The flow of reactant gas and capillary forces may cause the condensation to flow from the inlet manifold into the flow channels. To remove the accumulated water, the flow rate of the reactants through the fuel cell assembly or pressure drop across each fuel cell plate may be increased. However, increasing the flow rate or pressure drop decreases the efficiency of the fuel cell system.
Furthermore, the water accumulated on the fuel cell plates may form ice in the fuel cell assembly. The presence of water and ice may affect the performance of the fuel cell assembly. During typical operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the assembly and militates against vapor condensation and ice formation in the assembly. During a starting operation or low power operation of the fuel cell assembly in freezing temperatures, the condensed water in the flow channels of the fuel cell plates and at edges of the outlet manifolds may form ice within the fuel cell assembly. The ice formation may restrict reactant flow, resulting in a voltage loss and inefficient operation of the fuel cell system.
To further facilitate the removal of water, some fuel cell assemblies utilize plates having hydrophilic coatings or hydrophilic structures such as a foam, a wick, or a mesh. Water has been observed to form a film on the surface of the hydrophilic material. The film tends to accumulate at the outlet of the flow channels and the perimeter of the plates. The water film can block the gas flow, which in turn reduces the driving force for removing liquid water and thus militates against the removal of the liquid water from the fuel cell assembly. In the case of a fuel cell plate with a mildly hydrophobic surface, water has been observed to form large drops that protrude into the fuel cell assembly outlet manifold blocking the exits of the channels of the fuel cell plates. The droplets are observed to remain at the plate edge until they can be intermittently removed by gas shear. The accumulation of water can cause gas flow blockages or flow imbalances that may cause the fuel cell assembly to operate inefficiently. Fuel cell plates having a hydrophilic coating may be expensive to produce. Typically, the hydrophilic coating is disposed on the fuel cell plate using vacuum methods such as the plasma enhanced chemical vapor deposition (PECVD) method, the sol-gel method, and the atomic layer deposition (ALD) method. The use of the hydrophilic foam, the hydrophilic wick, or the hydrophilic mesh increases the material costs, assembly costs, and assembly time of the fuel cell assembly.
It would be desirable to develop a fuel cell plate for a fuel cell assembly with an improved means for removing liquid water from the flow channels of the fuel cell plate to minimize the accumulation of liquid water within the fuel cell assembly.
Concordant and congruous with the present invention, a fuel cell plate for a fuel cell assembly with an improved means for removing liquid water from the flow channels of the fuel cell plate to minimize the accumulation of liquid water within the fuel cell assembly has been discovered.
In one embodiment, a fuel cell plate comprises a plate having an inlet aperture and an outlet aperture formed therein and a plurality of flow channels formed between and in fluid communication with the inlet aperture and the outlet aperture; a hydrophobic portion formed on the flow channels adjacent the outlet aperture; and a hydrophilic portion formed on the flow channels adjacent said hydrophobic portion and forming an interface therebetween, wherein said hydrophobic portion and said hydrophilic portion facilitate a transport of water from the flow channels to the outlet aperture.
In another embodiment, the fuel cell plate comprises a plate having an inlet aperture and an outlet aperture formed therein and a plurality of flow channels formed between and in fluid communication with the inlet aperture and the outlet aperture; a hydrophobic portion formed on at least a portion of the flow channels adjacent the inlet aperture and the outlet aperture; and a hydrophilic portion formed on the flow channels adjacent the hydrophobic portion and forming an interface therebetween, wherein the hydrophobic portion and the hydrophilic portion facilitate a transport of water away from the flow channels.
In another embodiment, the fuel cell stack comprises a plurality of fuel cell plates, each of said plates having an inlet aperture and an outlet aperture formed therein and a plurality of flow channels formed between and in fluid communication with the inlet aperture and the outlet aperture; a hydrophobic portion formed on the flow channels adjacent at least the outlet aperture and the inlet aperture; and a hydrophilic portion formed on the flow channels adjacent said hydrophobic portion and forming an interface therebetween, wherein said hydrophobic portion and said hydrophilic portion facilitate a transport of water from the flow channels to the outlet aperture.